The Shale Gas Accumulation Conditions And Distribution Characteristics Of Black Shales In The Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation Of Northern Guizhou

This thesis is focused on the study of black shales from the Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation.I collected a wide variety of information from geological sections and drilling wells,as well as data derived from lab analysis,aiming to clarify lithological classification and naming system of organic-rich mudstones,and reveal organic geochemical properties,reservoir characteristics,and controlling factors of shale gas preservation.Based on this study,the preservation conditions and distributional pattern of shale gas in this area have been investigated.(1)The grain size analysis of Wufeng Formation-Longmaxi Formation black shales in northern Guizhou shows that mudstone is the main lithological type of the Upper Ordovician-Lower Silurian black shales in northern Guizhou.In order to establish a new lithological classification and naming system that can be used in the black shales of the Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation in northern Guizhou,mineralogical composition analysis has been conducted.Based on previous naming standards,the ration of organic carbon,quartz and feldspar,and authigenic mineral and clay have been considered as the controlling factors for lithological division.(2)According to the distributional pattern of rock association,sedimentary structure and biological assemblages,the micro-facies of Upper Ordovician-Lower Silurian black shales in northern Guizhou can be recognized.The lithofacies and paleogeographic maps with micro-facies can be drawn.These maps illustrate the favorite micro-facies for gas accumulation.(3)The distributional pattern of the Wufeng Formation-Longmaxi Formation organic-rich shales in northern Guizhou has been illustrated in this study.The organic-rich mudstones are mainly located in the lower part of the formation,and generally,the thickness of organic-rich mudstones is less than 20 m.The thickness of organic-rich mudstones becomes thicker northwards.It can reach to 60-100 m in Nanchuan-Wulong area that has a great potential for shale gas exploration.(4)The results of organic geochemical analysis of this study show that organic-rich mudstones of Longmaxi Formation are mainly distributed in northern area of Zheng'an-Wuchuan county.The organic carbon content in these mudstones ranges largely from 2% to 6%.Organic particles are characterized by the domination of amorphous and detrital sapropel components.Kerogen type is mainly type I,with minor type II.Ro values that indicate thermal maturity range 1.5%~2.2%.Shale gas is mainly cracked gas and dry gas.Ro values are correlated with the location of sedimentary center.These characteristics indicate that the organic-rich mudstones of Wufeng Formation-Longmaxi Formation in the northern area of northern Guizhou have a great potential for the shale gas generation.(5)Reservoir test shows that shale gas reservoir of Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation of the northern Guizhou is characterized by a high content of brittle minerals,high specific surface area,low porosity-super low permeability,and relatively well-developed micro-pores and fractures.Via the experimental study of reservoir wettability and sensitivity and rock mechanics,I analyzed the potential damage and rock mechanical parameters in the process of the reservoir transformation.The hydrophilic,middle-strong water sensitivity,and strong acid sensitivity of organic mudstones from the Wufeng Formation-Longmaxi Formation is due to the weak reservoir permeability and high capillary pressure.The fracture system should be optimized before the reservoir transformation.(6)The in-situ test demonstrated that organic mudstones from the Wufeng Formation-Longmaxi Formation are rich with gas.The gas density is generally higher than 1m3/t,with a maximum value of 4m3/t.When the onsite gas emission rate is higher,the in-situ logging signal is stronger.The onsite gas emission rate is related with the development of the micro-fractures of rock core.The higher onsite gas emission rate indicates a higher content of free shale gas in the targeted strata.This result shows that the sweet spot of the shale gas exploration is the strata with well developed micro-fractures.(7)This thesis systematically discussed factors controlling the shale gas preservation system,including the regional distributional pattern of the cover strata,the development of faults,the ground water type,the distributional pattern of hot springs,and the distributional pattern of epithermal deposits.My investigations show that the Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation of the northern Guizhou has a great potential for the shale gas exploration.The sweet spot of the shale gas exploration is targeted on the exposed Permian-Triassic strata of the synclinori?m that is far away from the deep faults,distributed in the northern area of Zhenga-Wuchuan-Yanhe.(8)Finally,I conclude that the shale gas accumulation in the organic mudstones from the Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation of the northern Guizhou is mutually controlled by the mineral components,organic carbon content,rock density,and rock physical properties.